Diarylsilanes

In a continuing study of substituent effects on the spectra of polysilane derivatives, we have succeeded in the preparation of the first soluble poly(diarylsilane) homopolymers. Materials of this type have traditionally proved to be insoluble and intractable. Very recently, West and co-workers have reported the preparation of some soluble copolymers which contain diphenylsilylene units (48,49). 

Since for the diarylpolysilanes, as for all polysilanes, the main chain is chromophoric due to the electronic transition between the delocalized silicon a and a orbitals, the electronic spectroscopies of CD, UV-Vis, and FL are particularly powerful probes of the structures of these materials. The magnitude of dimensionless quantity, gabs, is perhaps the most useful in comparing the CD spectra of optically active poly(diarylsilane)s. 

Chart 4.9 Chemical structure of optically active poly(diarylsilane) copolymers. 

Photoluminescence (PL) in the polysilanes is well documented,34b,34c and for the poly(diarylsilane)s occurs typically with a small Stokes shift and almost mirror image profile of the UV absorption.59 This is due to the similarity of the chromophore and fluorophore structures in the ground and excited states, respectively, which is a result of the fact that little structural change occurs on excitation of the electrons from the a to the a orbitals. As PL is the emissive counterpart to UV, the emissive counterpart to CD is circularly polarized pho-toluminescence (CPPL). Where the fluorophore is chiral, then the photoexcited state can return to the ground state with emission of circularly polarized light, the direction of polarization of which depends on the relative intensities of the right-handed and left-handed emissions (/R and /l, respectively), which in turn depends on the chirality of the material, or more accurately, the chirality... 

The UV and CD spectra of aggregates of 111, poly[bis-/>- -butylphenylsilylene0.5- -bis-/>-(V)-2-methylbutylphe-nylsilyleneo.5] (see above), in a toluene/methanol 40/60 solution are shown in Figure 50, compared with the solution spectra, and are typical of the spectra obtained for poly(diarylsilane)s under such conditions.340... 

IR bands of polysilane radical anions and the location of conduction electrons. Except for poly(diarylsilanes), the conduction electron is confined either in the skeleton or in one of the pendant groups. 

Diarylsilanes react with [Rh(/u.-H)(dippe)]2 to form [Rh(/x-SiAr2)(dippe)]2, but with dimethylsilane the product is (32), which is a catalyst precursor for the dimerization of diphenylsilane (see Silicon Inorganic Chemistry). The initial reaction of [Cp2Rh2(/u.-CH2)2(M-SH)]BPli4 with dimethylacetylene dicarboxylate is the insertion of one aUcyne into the S-H bond but the second equivalent of aUcyne couples with both methylene groups to form (33)." ... 

Alcoholysis of diarylsilanes." Tris(triphenylphosphme)chlororhodium and di-chlorotris(triphenylphosphine)ruthenium are effective catalysts for alcoholysis of diarylsilanes ... 

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